Simulcast transmission in wireless messaging systems is frequently utilized for coverage of large areas and to enhance the reliability of message delivery. An objective of simulcast transmission is to transmit multiple signals from multiple transmitters such that the information carried by the multiple signals arrives in synchronism at a receiver. In general, synchronism has been attempted with some success in prior art systems by adjusting either a delay or a launch time of the multiple transmitters such that the information leaves the transmitters substantially simultaneously.
Modern two-way messaging systems have made it possible to determine the location of subscriber units operating in the system. This also makes it possible to perform multiple directed simulcast transmissions using small, isolated clusters of cells, thereby allowing frequency reuse.
While frequency reuse is highly desirable, coordination of the multiple directed simulcast transmissions adds significantly to the processing burden of the messaging system controllers. Another problem that can occur during any simulcast transmission is delay spread. Delay spread occurs when a receiver is not equidistant from two or more simulcasting transmitters and is caused by differential transmission delay resulting from the unequal path lengths between the receiver and the transmitters. Delay spread limits the maximum possible symbol rate that can be utilized in a simulcast system, thereby limiting throughput.
Thus, what is needed is a method and apparatus in a wireless messaging system that can facilitate the dynamic creation of directed simulcast zones without adding significantly to the processing burden of the messaging system controllers. The method and apparatus preferably also will reduce delay spread in a simulcast system.